This invention relates to a method for improving the extraction properties of a solution of tributyl phosphate in an organic solvent. More specifically, this invention relates to a method for improving the extraction properties of a solution of tributyl phosphate by esterifying the chemical and radiological degradation products of tributyl phosphate which may be present therein.
As the numbers of nuclear reactors increase, the methods and techniques for reprocessing the spent fuel which has been irradiated in these reactors must also be improved. This is necessary to speed up reprocessing and to improve recovery of uranium and plutonium so that these and other reactors under construction and planned for in the future may be fueled.
A number of methods of fuel reprocessing have been developed such as halide volatility processes, pyrometallurgical processes and solvent extraction processes.
Of these processes, the solvent extraction process which utilizes tributyl phosphate (TBP) in a kerosene base is presently in wide use. This process is generally referred to as the Purex Process and is described in detail in "Reactor Handbook", Second Edition, Vol. 2, "Fuel Reprocessing", Stoller and Richards, Interscience, 1961. This process relies upon the extractability of uranyl nitrate and the relative inextractability of plutonium (III) to separate these two elements from each other and from an aqueous nitric acid feed solution.
The use of tributyl phosphate as an extractant for uranium and plutonium has had widespread acceptance in the nuclear industry for the past 25 years. This material, nevertheless, suffers from both chemical and radiological degradation when exposed to the radioactive solutions containing extractable actinides.
The presence of the degradation products such as the dibutyl phosphate (DBP), mono-butyl phosphate (MBP) and phosphoric acid has a detrimental effect upon the extraction process. Among the problems created by the presence of these materials are poor phase separation in the contractor, with subsequent losses of entrainment and/or reduced throughput, extraction of fission products along with the actinides, resulting in a loss of purity of uranium and plutonium, loss of actinides caused by the formation of nonstrippable complexes in the organic phase, and the formation of precipitates.
Thus it can be seen that the presence of degradation products seriously reduces the effectiveness of the extraction process. Techniques have been established to remove the degradation products. These include a carbonate aqueous scrub of the organic TBP solution, the use of macroreticular ion exchange resins and others. All of these techniques involve the use of extra columns for the cleanup, extra chemicals, a restoration of the cleanup system and finally the disposal of the additional radioactive waste stream. Even with the use of carbonate scrubs, periodically, the concentration of degradation products rises to a level such that the entire organic stream must be discarded. All of these treatment techniques are expensive. An ideal technique would be one that would restore the extracting solution either to its initial condition or one that closely approximates the initial solution.